This invention relates generally to medical devices and, more particularly, to devices for deploying another medical device such as a stent into a patient or for expanding a narrowed or obstructed passage or lumen in a patient.
Medical devices which incorporate inflatable or expandable balloons serve a wide variety of purposes. The balloon is carried on or affixed to a catheter shaft for delivery of the balloon to a desired location in the patient. The catheter shaft includes a lumen for introducing an inflation fluid into the balloon. For example, such catheter balloons are widely known to be useful for performing angioplasty procedures or the like, in which narrowings or obstructions in blood vessels or other body passageways are altered in order to increase blood flow through the narrow or obstructed area. More specifically, in a typical balloon angioplasty procedure, a balloon catheter is percutaneously introduced into the patient by way of the arterial system and advanced until the balloon of the catheter lies across the vascular narrowing or obstruction. The balloon is then inflated to dilate the vessel lumen at the site of the narrowing or obstruction. If desired, a stent may be positioned over the balloon and deployed at the site of the narrowing or obstruction to ensure that the dilated vessel lumen remains open. Balloon catheters find utility in a wide range of procedures, including valvuloplasty and urological procedures, among others.
The balloons of prior balloon catheters have been constructed from a wide variety of polymeric materials. These balloons each have their own advantages and drawbacks. Balloons comprising polyethylene terephthalate (PET), for example, have a relatively low degree of distention or expansion once they are inflated. This generally minimizes any potential adverse effects from overinflation or overexpansion of the balloon or any stent carried on it. Semi-distending or non-distending balloons often possess relatively high tensile strength, burst pressure and puncture resistance, qualities highly desirable for dilating tough lesions or for deploying and expanding stents carried over them.
However, body vessels such as arteries are generally tapered, and the locations at which narrowings or obstructions may occur vary, so that a balloon which closely matches the ultimately desired diameter of the vessel may not be readily available. Moreover, it may at times be desirable to be able to increase the diameter of the balloon beyond that which had been contemplated before the balloon procedure was begun. While balloons comprising materials such as polyvinyl chloride can be more distensible than PET or the like, balloons comprising such materials often possess a significantly lower tensile strength, burst pressure or puncture resistance than the less-distensible balloons. Overinflation of such balloons is also possible.
A variety of attempts have been made to construct medical device balloons from materials which yield balloons of good strength (that is, relatively high tensile strength and burst pressure, and good puncture resistance) while retaining an adequate degree of compliance, that is, an acceptable ratio of balloon diameter growth under an applied pressure to that balloon pressure. Each of these attempts possesses its own advantages and disadvantages. Balloons made from materials such as PET may possess excessive crystallinity or may be too stiff, so that such balloons may be resistant to the folding desired to minimize the profile of the catheter in which the balloon is employed; such resistance to folding is particularly problematic when the balloon is deflated following inflation during an in situ application, in order to be retracted into the distal end of the catheter for withdrawal. A minimal catheter profile is a highly desirable characteristic of balloon catheters, however. Some materials do not readily accept coating with drugs or lubricants, and some materials are difficult to fuse or adhere to conventional catheter shafts. Balloons made of some biaxially oriented nylons or polyamides have been asserted to overcome some of these problems.
Catheter balloons comprised of block copolymers have been suggested as a way of achieving an acceptable combination of balloon strength and elasticity. For example, it is known that catheter balloons can be constructed from polyamide/polyether block copolymers, commonly identified by the acronym PEBA (polyether block amide). Many of such copolymers can be characterized by a two phase structure, one being a thermoplastic region that is primarily a polyamide, semicrystalline at room temperature, and the other being an elastomer region that is rich in polyether. Balloons comprising such copolymers are asserted to possess a desirable combination of strength, compliance and softness. Catheter balloons comprising blends of two or more such copolymers are also known, and it has been asserted that irradiating such blends can enhance the properties of the resulting balloons, including increased burst pressures.
It would be highly advantageous to have medical devices which included expandable or inflatable balloons with improved strength, for example, with greater tensile strength, burst pressure and/or puncture resistance, while simultaneously possessing acceptable compliance (in this case, an acceptable ratio of balloon diameter growth to balloon pressure). It would also be highly advantageous to have medical devices made from materials which meet a variety of desirable processing criteria, including thermal stability, non-toxicity, non-volatility, high boiling point (preferably, solid at room temperature), high flash point, insensitivity to moisture and commercial availability.
Many of the foregoing problems are solved and a technical advance is achieved in an illustrative medical device for positioning an included balloon within a human or veterinary patient, for example, for deploying another medical device such as a stent in the patient or for expanding a passage or lumen in the patient. More particularly, in a first preferred embodiment, the medical device of the present invention comprises a catheter shaft and an expandable balloon carried by the catheter shaft. The medical device of the present invention is characterized in that the balloon comprises an irradiation cross-linked mixture of a polyamide elastomer and at least one additional cross-linking reactant.
This additional cross-linking reactant performs a role which is quite different from that performed by the two reaction promoters disclosed in International Application WO 98/55171. That Application is directed to a cross-linked nylon block copolymer which comprises an irradiation cross-linked copolymer containing a polyamide block and an elastomeric block, including a compound which promotes cross-linking therein. The process disclosed in that application comprises supplying the nylon block copolymer with a cross-linking xe2x80x9cpromotorxe2x80x9d (sic.) and exposing the block copolymer to irradiation sufficient to cross-link the copolymer. Only two promoters are disclosed, triallylcyanurate and triallylisocyanurate, at 2 percent by weight in PEBAX(copyright) brand nylon block copolymer (Atochem, Inc., brand of polymers consisting of polyether blocks separated by polyamide blocks). Irradiation is carried out at 5 to 20 megarads (no specific type of irradiation is disclosed), although degradation of the material may take place when total irradiation becomes too high, for example, at 15 or 20 megarads. That Application claims (among others) an improvement in a balloon type catheter having a tubular shaft comprising a nylon block copolymer and an integrally formed balloon section, the improvement comprising irradiation crosslinking the copolymer of the balloon section, wherein the crosslinking lowers the percent elongation of the balloon section as compared to the elongation prior to crosslinking. The only apparent support in the specification for that claim appears to be a single statement that, in the case of balloon catheters manufactured from a nylon block copolymer, the invention therein provides for the preparation of a balloon type catheter wherein the balloon section relative to the shaft can be converted into a thermoset or crosslinked type structure, thereby increasing its overall mechanical strength, performance, and durability. That Application appears to make no other disclosure of any process whatsoever for manufacturing such a balloon, and appears to contain absolutely no details as to how such a process could or should be carried out.
The present invention is quite distinct; the cross-linking reactant of the present invention and the promoter of that Application appear to act in different ways to perform different functions. xe2x80x9cPromoterxe2x80x9d is a well-recognized term of art, of course, referring to a material which enhances the activity of a catalyst. More particularly, a promoter is a substance that, when added in relatively small quantities to a catalyst, increases its activity; Lewis, Sr., Hawley""s Condensed Chemical Dictionary 12th (Van Nostrand Reinhold Company, New York, N.Y., 1993) (definition 1), at 966; or is a chemical which itself is a feeble catalyst, but greatly increases the activity of a given catalyst; Parker, McGraw-Hill Dictionary of Scientific and Technical Terms 5th (McGraw-Hill, Inc., New York, N.Y., 1994) (first definition), at 1589. Catalysts, of course, accelerate or retard the velocity of a chemical reaction without being consumed during the course of those reactions. They do not become incorporated into the chemical structures of the products of the reactions, and in theory can be recovered at the end of the reaction essentially unaltered in form and amount (even though in practice they might be retained in the physical object constituted by the reaction products). This is presumably true of the two materials mentioned in that Application, since they appear to be solely described in that Application as xe2x80x9cpromoters.xe2x80x9d While it might be argued whether energy should properly be called a catalyst, it is believed that the use of the word xe2x80x9cpromotersxe2x80x9d in that Application would be readily understood by those in the medical device field to refer to materials which increased the activity of the irradiation employed in that Application, that is, increased irradiation cross-linking between the chains themselves of the nylon block copolymer it discloses.
In direct contrast to any balloon or medical device containing the two specific promoters of that Application at their disclosed concentrations, the medical device of the present invention comprises a balloon in which one or more specific cross-linking reactants are, by irradiation, chemically incorporated into the polyamide elastomer with which they are initially mixed. Thus, where the two promoters of that Application would cause the various chains within the polyamide elastomer of any balloon to cross-link directly to one another, the specific cross-linking reactants in the balloon of the present invention instead themselves form and constitute links or bridges between the various chains within the polyamide elastomer. Thus, the molecular structure and physical properties of the balloon incorporated in the medical device of the present invention are different from those which might be expected to be possessed by a balloon which included either of the two catalysts or promoters of that Application.
The particular cross-linking reactants useful in the medical device of the present invention, and in particular, in the balloon thereof, are expected to include difunctional materials such as diallyl adipate; diallyl carbonate; diallyl maleate; diallyl succinate; diallyl tetrabromophthalate; diethyl diallylmalonate; dimethyl diallylmalonate; and 2,2,6,6-tetrabromo-bisphenol A diallyl ether. Useful cross-linking reactants are also expected to include trifunctional materials such as 2,5-diallyl-4,5-dimethyl-2-cyclopenten-1-one; diallyl fumarate; diallyl itaconate; 1,3,5-triallyl-2-methoxybenzene; triallyl trimesate (triallyl 1,3,5-benzenetricarboxylate); triallyl trimellitate (triallyl 1,2,4-benzenetricarboxylate); and pentaerythritol triallyl ether; and tetrafunctional materials such as tetraallyl cis,cis,cis,cis-cyclopentane-1,2,3,4-tetracarboxylate; and N,N,Nxe2x80x2,Nxe2x80x2-tetraallylethylenediamine. Useful materials are also expected to include aromatic molecules containing at least two ring substituents, each of the ring substituents having labile hydrogens at a benzylic site therein. 1,3,5 triethyl benzene; 1,2,4 triethyl benzene; and 1,3,5 triisopropyl benzene are commercially available examples of such aromatic molecules containing at least two substituents having labile hydrogens at a benzylic site. Useful materials are further expected to include diallyl phthalate and meta-phenylene dimaleimide; these latter two constitute a second preferred embodiment of the present invention.
All of these materials are expected to possess at least several of a variety of desirable characteristics for manufacturing the medical device of the present invention: thermal stability, non-toxicity, non-volatility, high boiling point (preferably, solid at room temperature), high flash point, insensitivity to moisture and commercial availability. However, not all of these materials possess all of these desirable characteristics. Other materials capable of forming allylic or benzylic radicals having comparable reactivity should be useful as well. The primary criteria for selecting such other materials may be that they are less reactive than species such as epoxies, methacrylates and acrylates; and that they are relatively xe2x80x9csmallxe2x80x9d molecules, that is, they are small enough to fit between (and thereby be capable of cross-linking) the various chains of the particular polyamide elastomer being used. The materials must of course be multifunctional, to be able to cross-link to at least two of those chains.
The more reactive species such as epoxies, methacrylates and acrylates are probably undesirable for use in the medical device of the present invention because they are likely to cross-link the polyamide elastomer too rapidly, completing the cross-linking reaction during preliminary thermal processing of the polyamide elastomer (prior to its being formed into the balloon of the device). Such premature cross-linking clogs the processing equipment, such that completion of the balloon-forming process is impossible. Multifunctional allylic materials are more stable and less reactive than these, so that they readily survive thermal processing but are still reactive enough when exposed to a source of energy to achieve good cross-linking.
The allylic radical and the benzylic radical differ in bond dissociation energies (and hence radical stabilities and reactivities) by only 2 kcal/mol (7 kJ/mol); J. March, Advanced Organic Chemistry 4th, at 191 (John Wiley and Sons, New York, N.Y., 1992). Accordingly, a wide variety of multifunctional benzylic small molecules are expected to be useful in the medical device of the present invention; the three listed above have the advantage of being commercially available at the present time. The selection of other materials having suitably positioned labile hydrogens should be well within the level of skill in the field of designing medical devices of this type, since the recognition of labile hydrogen positions is generally taught quite early in introductory (college sophomore) organic chemistry.
While some modest degree of trial-by-error experimentation may be needed to confirm the practical utility of any particular allylic or benzylic material contemplated for use in the present invention but not specifically disclosed herein, such experimentation is not believed to be undue under the circumstances, but is instead believed to be substantially below the amount of testing that would be required for FDA approval for actually marketing a medical device incorporating a balloon comprising such a particular material as a cross-linking reactant.
While attempting to manufacture a medical device balloon employing the two promoters disclosed in International Application WO 98/55,171, it was discovered that these two specific materials could in fact act as cross-linking reactants (instead of merely augmenting the cross-linking activity of the disclosed irradiation) under concentrations or conditions other than the concentrations or conditions disclosed in that Application. More particularly, attempts to make a parison for forming a medical device balloon from a mixture of PEBAX(copyright) brand nylon block copolymer with 2 percent by weight of one of those materials were generally unsuccessful or unacceptable for commercial purposes, due to the significant formation of gelling in the parison. xe2x80x9cGellingxe2x80x9d is a term of art which indicates the formation of small, discrete volumes, areas, particles or particulates which are a result of premature, undesirable thermal cross-linking of the copolymer or other polyamide elastomer itself. xe2x80x9cGellingxe2x80x9d also includes other defects arising during the manufacture of the copolymer or other polyamide elastomer. Gelling in the particular mixture under consideration prevented the successful use of the resulting parison to form a balloon for commercial purposes.
Since that Application teaches that higher levels of irradiation are undesirable, it is believed that those skilled in the field would have concluded that the only alternative left for improving the amount of cross-linking would have been to increase the amount of promoter mixed with the copolymer. Efforts in this direction were unsuccessful. Unexpectedly, it was found that an acceptable balloon could be obtained by lowering, not increasing, the amount of the promoter. As a result, gelling was decreased to an acceptable level. It was found that at these lower levels the so-called xe2x80x9cpromoterxe2x80x9d itself acted as a cross-linking reactant, incorporated in the structure of the cross-linked copolymer between the chains of the copolymer. Such a result appears to be directly contrary to any reasonable expectation from the disclosure of that Application.
Accordingly, in a third preferred embodiment, the medical device of the present invention comprises a combination which is comparable to the first preferred embodiment, but which is instead characterized in that its balloon is formed from an irradiated mixture of a polyamide elastomer and no more than about 1.5 percent by weight of either triallyl cyanurate or triallyl isocyanurate. It is believed that these two materials advantageously possess most or all of the desirable characteristics mentioned above.
In all of these embodiments of the present invention, the polyamide elastomer can be one or more members of any of the three generally recognized families of polyamide elastomers: polyester amides (or PESAs), polyether ester amides (PEEAs) or polyether amides (PETAs). Representative PESAs include ESTAMID(copyright) brand polymer from Dow Chemical Company. Representative PEEAs include PEBAX(copyright) brand nylon block copolymer, VESTAMID(copyright) brand polymer from Creanova Corporation and GRILAMID(copyright) brand polymer from Esmer Corporation. Representative PETAs include GRILON(copyright) brand polymer, also from Esmer Corporation.
Other preferred embodiments of the present invention described in more detail below include the processes by which these three embodiments of the medical device of the present invention are assembled. The medical device of the present invention may be particularly advantageous in that the puncture resistance, strength and burst pressure of its balloon may be improved with respect to comparable irradiation cross-linked balloons lacking any cross-linking reactant.
In a first aspect, then, the present invention is directed to a medical device comprising: a catheter shaft; and an expandable balloon carried by the catheter shaft; wherein the balloon comprises an irradiation cross-linked mixture of a polyamide elastomer and at least one additional cross-linking reactant, the cross-linking reactant comprising: (a) a difunctional material selected from the class consisting of diallyl adipate; diallyl carbonate; diallyl maleate; diallyl succinate; diallyl tetrabromophthalate; diethyl diallylmalonate; dimethyl diallylmalonate; and 2,2,6,6-tetrabromobisphenol A diallyl ether; (b) a trifunctional material selected from the class consisting of 2,5-diallyl-4,5-dimethyl-2-cyclopenten-1-one; diallyl fumarate; diallyl itaconate; 1,3,5-triallyl-2-methoxybenzene; triallyl trimesate (triallyl 1,3,5-benzenetricarboxylate); triallyl trimellitate (triallyl 1,2,4-benzenetricarboxylate); and pentaerythritol triallyl ether; (c) a tetrafunctional material selected from the class consisting of tetraallyl cis,cis,cis,cis-cyclopentane-1,2,3,4-tetracarboxylate; and N,N,Nxe2x80x2,Nxe2x80x2-tetraallylethylenediamine; or (d) an aromatic molecule containing at least two ring substituents, each of the ring substituents having labile hydrogens at a benzylic site therein. In a second aspect, the present invention is directed to such a device in which the at least one additional cross-linking agent comprises diallyl phthalate or meta-phenylene dimaleimide.
The balloon of the medical device preferably comprises an amount of the cross-linking reactant sufficient to give the balloon a strength generally about equal to and perhaps in some cases greater than that of a balloon composed of the polyamide elastomer and comparably cross-linked by irradiation, but in the absence of any cross-linking reactant, agent or promoter. The balloon more preferably comprises about 1 to about 2 percent by weight of the difunctional material; about 0.5 to about 1.5 percent by weight of the trifunctional material or the aromatic molecule containing at least two ring substituents, each of the ring substituents having labile hydrogens at a benzylic site therein; or about 0.01 to about 1 percent by weight of the tetrafunctional material. The balloon alternatively comprises about 1 to about 2 percent by weight diallyl phthalate or meta-phenylene dimaleimide.
The balloon of the medical device further preferably comprises a mixture of the polyamide elastomer and the cross-linking reactant which has been cross-linked by irradiation with an electron beam or with ultraviolet, X- or gamma rays. More preferably, the balloon comprises a mixture of the polyamide elastomer and the cross-linking reactant which has been cross-linked by exposure to about 0.5 to about 20 megarads of radiation. It is preferred that the balloon is formed by inflation of the mixture of the polyamide elastomer and the cross-linking reactant after the mixture has been cross-linked by irradiation.
As indicated above, the balloon of the medical device can comprise any member of the polyamide elastomer families, such as polyester amides, polyether ester amides or polyether amides. The balloon preferably comprises a nylon block copolymer including polyamide blocks separated by elastomeric polyether blocks or segments. Suitable nylon block copolymers of this type are sold under the trademark PEBAX(copyright) by Atochem, Inc. Useful nylon block copolymers can instead include polyamide blocks separated by other elastomeric blocks or segments, such as polyesters, hydrocarbons or polysiloxanes.
When the balloon comprises an irradiation cross-linked mixture of a polyamide elastomer and an aromatic molecule, it is preferred that the aromatic molecule containing at least two ring substituents, each of the ring substituents having labile hydrogens at a benzylic site therein, is selected from the class consisting of 1,3,5 triethyl benzene; 1,2,4 triethyl benzene; and 1,3,5 triisopropyl benzene.
In a third aspect, the present invention is directed to a medical device comprising: a catheter shaft; and an expandable balloon carried by the catheter shaft; wherein the balloon comprises an irradiation cross-linked mixture of a polyamide elastomer and no more than about 1.5 percent by weight of at least one additional cross-linking reactant, the cross-linking reactant comprising triallyl cyanurate or triallyl isocyanurate. Preferably, the balloon comprises an amount of the cross-linking reactant sufficient to give the balloon a strength generally about equal to and in some cases perhaps greater than that of a balloon composed of the polyamide elastomer and comparably cross-linked by irradiation, but in the absence of any cross-linking reactant, agent or promoter.
In this third aspect, the balloon of the medical device preferably comprises a mixture of the polyamide elastomer and the cross-linking reactant which has been cross-linked by irradiation by an electron beam or by ultraviolet, or X- or gamma rays. Even more preferably, the balloon comprises a mixture of the polyamide elastomer and the cross-linking reactant which has been cross-linked by exposure to about 0.5 to about 20 megarads of radiation. The balloon is preferably formed by inflation of the mixture of the polyamide elastomer and the cross-linking reactant after the mixture has been cross-linked by irradiation.
As in the first aspect of the present invention, the balloon of the medical device of the second and third aspects of the present invention preferably comprises a polyester amide, a polyether ester amide or a polyether amide, and more preferably comprises a nylon block copolymer including polyether blocks separated by polyamide blocks, such as PEBAX(copyright) brand nylon block copolymer.
In a fourth aspect, the present invention is directed to a process for assembling a medical device, the medical device comprising an expandable balloon, and the process comprising: creating a mixture of a polyamide elastomer and at least one additional cross-linking reactant, the cross-linking reactant comprising: (a) a difunctional material selected from the class consisting of diallyl adipate; diallyl carbonate; diallyl maleate; diallyl succinate; diallyl tetrabromophthalate; diethyl diallylmalonate; dimethyl diallylmalonate; and 2,2,6,6-tetrabromobisphenol A diallyl ether; (b) a trifunctional material selected from the class consisting of 2,5-diallyl-4,5-dimethyl-2-cyclopenten-1-one; diallyl fumarate; diallyl itaconate; 1,3,5-triallyl-2-methoxybenzene; triallyl trimesate (triallyl 1,3,5-benzenetricarboxylate); triallyl trimellitate (triallyl 1,2,4-benzene-tricarboxylate); and pentaerythritol triallyl ether; (c) a tetrafunctional material selected from the class consisting of tetraallyl cis,cis,cis,cis-cyclopentane-1,2,3,4-tetracarboxylate; and N,N,Nxe2x80x2,Nxe2x80x2-tetraallylethylenediamine; or (d) an aromatic molecule containing at least two ring substituents, each of the ring substituents having labile hydrogens at a benzylic site therein; cross-linking the mixture of the polyamide elastomer and the at least one additional reactant by exposing the mixture to a suitable fluence of radiation; and forming the cross-linked mixture into the balloon. In a fifth aspect of the present invention, this process is instead carried out with at least one additional cross-linking reactant comprising diallyl phthalate or meta-phenylene dimaleimide.
The process of the present invention for assembling the medical device is preferably carried out with an amount of the cross-linking reactant sufficient to give the balloon a strength generally about equal to, and perhaps in some cases greater than, that of a balloon composed of the polyamide elastomer and comparably cross-linked by irradiation, but in the absence of any cross-linking reactant, agent or promoter. It is also preferred that the process is carried out with an amount of the cross-linking reactant which, when mixed with the polyamide elastomer and processed, causes the mixture from which the balloon is made to lack appreciable gelling during processing prior to irradiation and cross-linking. More preferably, the process is carried out with a mixture comprising about 1 to about 2 percent by weight of the difunctional material; about 0.5 to about 1.5 percent by weight of the trifunctional material or the aromatic molecule containing at least two ring substituents, each of the ring substituents having labile hydrogens at a benzylic site therein; or about 0.01 to about 1 percent by weight of the tetrafunctional material. Alternatively, the process can be carried out with about 1 to about 2 percent by weight diallyl phthalate or meta-phenylene dimaleimide.
Cross-linking of the mixture of the polyamide elastomer and the at least one additional reactant preferably comprises irradiating the mixture with an electron beam or with ultraviolet, X- or gamma rays. Irradiation is more preferably carried out at a total fluence of about 0.5 to about 20 megarads.
The process of the present invention is preferably carried out with the polyamide elastomers described above. More preferably, the process of the present invention is carried out with a nylon block copolymer which includes polyether blocks separated by polyamide blocks, such as PEBAX(copyright) brand nylon block copolymer. When the process is carried out with an aromatic molecule containing at least two ring substituents, each of the ring substituents having labile hydrogens at a benzylic site therein, it is preferred that the molecule is selected from the class consisting of 1,3,5 triethyl benzene; 1,2,4 triethyl benzene; and 1,3,5 triisopropyl benzene. Without regard to the specific polyamide elastomer and the at least one additional reactant employed in the present invention, however, it is preferred that the mixing of them is carried out by compounding (including such steps as melting, mixing and extruding, for example) or by blending.
The process of the present invention for making a medical device preferably further comprises forming the mixture of the polyamide elastomer and the at least one additional reactant into tubing, from which the balloon is formed. It is further preferred that the tubing is formed by extruding the mixture of the polyamide elastomer and the at least one additional reactant. Most preferably, the mixture of the polyamide elastomer and the at least one additional reactant is then formed into the balloon by inflation of the tubing. The process of the present invention can further comprise connecting the balloon so formed to a catheter shaft, for example, by adhesion or thermal bonding.
In a sixth aspect, the present invention is directed to a process for assembling a medical device, the medical device comprising an expandable balloon, and the process comprising: creating a mixture of a nylon block copolymer and no more than about 1.5 percent by weight of at least one additional cross-linking reactant, the cross-linking reactant comprising triallyl cyanurate or triallyl isocyanurate; cross-linking the mixture of the polyamide elastomer and the at least one additional reactant by exposing the mixture to a suitable fluence of radiation; and forming the cross-linked mixture into the balloon.
Other than the use of these two specific cross-linking reactants at the specified amounts, the preferred details of carrying out the process of this sixth aspect of the present invention are very comparable to the details of carrying out the process of the fourth aspect of the invention. Most notably, cross-linking of the mixture of the polyamide elastomer and the at least one additional reactant preferably comprises irradiating the mixture with an electron beam or with ultraviolet, X- or gamma rays. Irradiation is more preferably carried out at a total fluence of about 0.5 to about 20 megarads. The balloon is preferably formed by inflation of a tubing extruded from the mixture of the polyamide elastomer and the at least one cross-linking reactant, the tubing being irradiated before the balloon is formed from it. The process of the sixth aspect of the present invention is most preferably carried out with a nylon block copolymer including polyether blocks separated by polyamide blocks, such as PEBAX(copyright) brand nylon block copolymer.
In a seventh and final aspect, the present invention is directed to a medical device comprising: a catheter shaft; and an expandable balloon carried by the catheter shaft; wherein the balloon comprises an irradiation cross-linked mixture of a polyamide elastomer and at least one additional cross-linking reactant. This aspect of the invention may instead be considered as an improvement in a medical device comprising a catheter shaft and an expandable balloon carried by the catheter shaft, characterized in that the balloon comprises an irradiation cross-linked mixture of a polyamide elastomer and at least one additional cross-linking reactant.
As indicated above, the medical device of the present invention possesses significant advantages over prior devices for dilating a narrowing or obstruction in a vessel or lumen in a patient, and for deploying a stent across the site of such a narrowing or obstruction to prevent its restenosis. The balloon of the device of the present invention has generally improved strength, for example, greater tensile strength, burst pressure and/or puncture resistance, while simultaneously possessing acceptable compliance, that is, an acceptable ratio of balloon diameter growth to balloon pressure. Gelling during its manufacture, if present, is limited to an acceptable level. The balloon of the device of the present invention is made from materials which meet a variety of desirable processing criteria, including thermal stability, non-toxicity, non-volatility, high boiling point (preferably, solid at room temperature), high flash point, insensitivity to moisture and commercial availability. A second polyamide elastomer or another polyamide (such as nylon) may be added in a minor amount (less than 50 percent by weight or mole fraction), but is not required.